Femtosecond lasers are well known and used for many different applications. For example, femtosecond lasers are frequently used for bio-medical applications such as confocal microscopy, flow cytometry, multiphoton, excitation microscopy; laser machining, such as engraving, marking, drilling, rapid prototyping; and general scientific applications such as fluorescence spectroscopy, LIDAR, molecular spectroscopy, and ultrafast spectroscopy.
Commercial ultrashort-pulse lasers are engineered towards an optimum in pulse spectral width, short pulse duration and power. However, typically the beam output from the laser has to pass through several external optical components before reaching a desired location at which an experiment is to be performed. This causes dispersion, resulting in spreading of the pulses in time. This dispersion causes the pulse to attain a phase shift of its frequency components, which is called a ‘frequency chirp’.
To overcome the problem of dispersion, a pulse compressor or an adaptive optics pulse shaper may be placed in the beam path from the laser. These apply the inverse of the phase shift or chirp to the pulse phase profile. This inversely chirped or ‘pre-chirped’ pulse is compressed when propagating through the relay optics and a dispersion-free pulse can thus be delivered to the experiment. Examples of suitable compressors include a prism pulse compressor, a grating pulse compressor and a dispersion-shifted optical fiber. Examples of pulse shapers that exploit adaptive optics include a liquid crystal-based spatial light modulator (SLM), pulse shaper and a deformable mirror pulse shaper.
Although the use of compressors and/or adaptive optics externally to the laser overcomes some of the problems of dispersion, doing so generally introduces a loss of power and adds complexity to the setup.